Peavey XXX massive distortion

[thrgadgetman]

I am trying to fix my friends XXX. It was working great then he blew an internal fuse. When we fired it back up the clean channel has massive distortion and the 2 other channels the sound doesn't come thru just squeels and some rf noise.

[defaced]

There's a reason the internal fuse blew, did you ever figure out why? What fuse is it? The schematic can be gotten from Peavey or there are good copies online.

[thrgadgetman]

First the fuse blew because a tube went out. He put a new one in on the outside, and it powered on, but no noise. I opened it up and replaced an internal fuse and everything worked for a while. This happened last several months so I can't remember what fuse it was. So it happened again 2 weeks later he did the same thing I replaced the tubes as it was obvious this time,cause the tube was silver and would blow fuses instantly. Everything is great again for 2 months then the same thing happens again I check everything and find a screen resistor on the third power tube has been rubbing against the chassis and you can see the wire thru the coating obviously shorting out. This fixes the fuse problem , but I can't figure out what else fryed because of this. I checked every resistor and the all read correct. I don't know how the check the capacitors without just taking them out and putting new ones in. I don't know how to check the relays. I have all major electronic tools (function generator,scope, and digital multimeter), but have no formal education in electronics. I have built over a dozen tube amps without a kit so I can figure stuff out if someone points me in the right direction.

[thrgadgetman]

Ok after several hours of testing and sending signals thru at different points the problem has been narrowed down to the output power tube card. If I short the signal after C104 it helps, but still a problem. Same if I short after C100 except that makes V5 glow red. Its not the output transformer, cause I switched it with a good one and had the same issue. Also the dampening switch makes a big difference. On loose the sustain and distortion go one forever even with a small signal.

[thrgadgetman]

Norrowing down the problem

Ok after several hours of testing and sending signals thru at different points the problem has been narrowed down to the output power tube card. If I short the signal after C104 it helps, but still a problem. Same if I short after C100 except that makes V5 glow red. Its not the output transformer, cause I switched it with a good one and had the same issue. Also the dampening switch makes a big difference. On loose the sustain and distortion go one forever even with a small signal.

[oc disorder]

If you're sure the screen resistors are ok check all the diodes on the output power board.

[thrgadgetman]

not that

Just got my 2 KV diodes today replaced all 4 diodes and all 4 screen resistors no change, but the amp seems to have a reaccuring them with V7 always running hotter than the rest of the tubes. It started to glow red all the way thru when I checked it this time.

[Enzo]

Um....

Pull ALL the power tubes and power up the amp. Measure DC voltage on ALL FOUR power tube sockets. There should be B+ on ALL pins 3 AND 4. It will be something like 480-500v. I don;t care about a little difference, but if one is missing that is a clue. Then ALL pin 5s, for about -55VDC. ANy missing? And with power off, measure RESISTANCE to ground from pin 8 of each socket. If any of that is missing, fix it before continuing.

The report on changing tubes is not clear, do we have a fresh set of FOUR KNOWN GOOD tubes in there now?

[thrgadgetman]

Thank You Enzo
All tubes are brand new. Pins 3,4 are all 496V pin 5 jumps from 0 to +3.5V resistance is .3 Ohms on pin 8

[Enzo]

Then find out why there is 0 to +3.5v instead iof -55v on pins 5. That is a missing bias supply one way or another.

[thrgadgetman]

R123 read right on the ohm meter, but voltage wasn't making it thru replaced it now get a reading on pin 5 of -44.3V. Strange observation if I check with the ground test point it reads 0 if I test with the ground on the red test point it reads -44.3. when I test with ground to chassis it reads -44.3. bias test point with black and red in proper location reads 45.1. None of this change anything V7 still runs hotter than the rest and still massive distortion on the clean channel and nothing is coming thru on the other two channels. If I check pin 8 with ground at black bias test point it rises from 10k to 17k if I check it to chassis ground then I get 0 or .1

[Enzo]

Wait. Stop using those bias supply test points. Unless otherwise specified, measure voltages to ground - and that means chassis. The bias test points are for setting bias voltage only.

If V7 runs hotter than the others, we want to know if it has a different voltage on its grid - pin 5 - than the other three. And if that is the same, then regardless of the tubes being new, swap V7 with another. If ANY tube in the V7 socket overheats, then the socket still has the issue. If the same tube gets too hot in teh new socket, then the tube is at fault.

Any power tube problems have nothing to do with preamp channel problems. They are a whole separate issue.

[thrgadgetman]

already tried that all tubes run hotter in V7 and its grid is the same voltage as the rest.

[Enzo]

You running 6L6s or EL34s?


If all four screen resistors are the same and correct, if all four 10k grid stoppers are the same and correct, and B+ is on all the screens, and bias is on the grid, then I see no reason for one tube to red plate without the others. We are overlooking something.

Turn off all the room lights - BE CAREFUL - and watch the V7 socket for any sign of arcing - a tiny blue spark. It could be top or bottom. And if some parts are not visible, listen very carefully. AN arc usually makes a little noise like a pressure leak. FSSSSSSS.

[thrgadgetman]

OK the V7 problem fixed it looks like the factory forgot to solder one end of the 10k resistor. Soldered it now it doesn't red plate any more, but didn't fix the problem. Running KT77's and replaced the 100 Ohm 5W Resistors with 720 Ohm 5W resistors. Bias is set as if I was running EL34's. One of these rubbed against the chassis wearing off the ceramic coating and shorted out which caused the fuse to blow and the current problem. I have replaced them all and made sure they will not touch the chassis now.